Control devices, elevator-systems, comprising control devices and methods for pressure control in hydraulic systems, as mentioned above, are known from the prior art. In a hydraulic elevator system, a motor is usually coupled to a screw-pump which produces an oil flow and pressure that is supplied to a cylinder through a control valve. As the ram (piston) moves, it pushes or pulls the car (cabin).
In order to have good ride-quality; smooth start, accurate acceleration and deceleration, as well as smooth stop are important properties to satisfy. Full and levelling (small) speeds are preferably kept unchanged regardless of the changes of elevator load and/or oil temperature. It is important to keep the elevator speeds (full and levelling) constant otherwise the complete travel time becomes longer, which causes uncomfortable ride-quality, poor stopping accuracy (bigger than ±10 mm), affects the traffic cycle and increases the energy consumption of the elevator. Unfortunately, elevator load and fluid temperature influence the leakage of the pump drastically which varies the speed and the total travel time of the hydraulic elevator.
Hydraulic elevator solutions according to the prior art that assure expected ride-quality by means of inverters are too costly and complicated to meet market expectations. They require not only a special control valve but also load and/or flow sensors, mostly closed loop control (requires expensive submersible encoder and necessary electronic interface), costly electronic boards and trained service personnel. Additionally, to increase speed compensation accuracy and avoid noise problems mostly low-leakage, less-noisy screw pumps are employed at the cost of increased initial costs of the system.
Moreover, in the last ten years, energy efficiency has become an important product specification. Especially in the European Union, directives and standards are being modified to cover up the energy efficiency criteria on all products, including elevators. According to a new building code, energy efficient building equipment is enforced. Hence, it is expected that soon energy efficient elevators will be made compulsory for buildings in order to obtain green-building certification, which exempts building owners from paying taxation.
Consequently, a large number of renovations of hydraulic elevators are expected to take place in the coming years. Additionally, invasion of high life standards into developing countries and the rest of the world gave rise to the standards of the European Union being targeted by many non-European countries. Therefore, a majority of new elevator installations is expected to have high energy efficient properties.
Today, the use of inverters for powering hydraulic pumps is regarded as the ultimate energy efficient solution for elevator-systems. However, solutions with inverters have been either too primitive to assure expected standards or too expensive and complicated to meet market expectations. Thus, hydraulic solutions with inverters for powering hydraulic pumps could not find a vast acceptance in the market, even though a demand for energy saving elevator technology is increasing as already mentioned.